Silicon to protect legumes from insect herbivores
The increased use of legumes in cropping systems is recognised as a key strategy to progress toward net-zero. Legumes are beneficial in this context because their symbiosis with nitrogen-fixing Rhizobium sp. bacteria provides nitrogen without the need for synthetic fertilizers. This biological nitrogen fixation therefore reduces the carbon footprint of cropping systems and benefits neighbouring or subsequent crops. Unfortunately, high concentrations of nitrogen in legume plant tissues make them more palatable to pests. Moreover, ongoing work to enhance the N-fixation benefits of legumes may inadvertently enhance their susceptibility to pests. As we seek to expand the use of legumes in global cropping systems, we need sustainable low-carbon footprint methods to protect them from herbivory.
The role of silicon (Si) accumulation in grasses as a key defence against vertebrate and invertebrate herbivores is now well established (Massey et al. 2006; Massey & Hartley 2006) but it has been suggested that it could also play a role in such anti-herbivore defences in legumes (Putra et al. 2020). Si applications may therefore provide a sustainable method to pest-proof legumes, as an alternative to increasing dependence on carbon-intensive and environmentally-damaging pesticide sprays. However, very little is currently known about whether Si can alleviate the impacts of herbivory in legumes (Putra et al. 2021).
This study aimed to better understand the role of Si in anti-herbivore defences in legumes, particularly in relation to jasmonic acid (JA), a phytohormone linked to Si accumulation that regulates anti-herbivore defences. We measured the growth, root nodulation and chemistry (carbon, nitrogen and phenolic concentrations) of four genotypes of a legume (lucerne or alfalfa, Medicago sativa) after plants were subjected to soil Si supplementation, with and without the application of JA. Feeding assays were subsequently performed to determine the effects of Si and JA on the feeding success of a common lucerne herbivore (the leaf-chewing adult weevil, Sitona discoideus).
Supplementing lucerne plants with Si resulted in a 61% increase in plant mass and a more than threefold increase in the number of root nodules. Despite this increase in nodulation, Si supplementation did not alter foliar nitrogen concentrations, which was likely due to the dilution effects of increased plant growth and higher foliar carbon concentrations. Si supplementation resulted in a 38% reduction in relative consumption (RC) and a 30% decrease in frass production by S. discoideus. Moreover, there was a negative correlation between phenolic concentrations and leaf RC, and when JA was applied to the plants, RC decreased by 34%. Combining Si with JA application resulted in a significant increase in phenolic compounds, demonstrating the potential of Si to stimulate various anti-herbivore defences in lucerne. These phytochemical changes and herbivore feeding responses were consistent across all four lucerne genotypes that were tested.
In summary, the study provides a novel insight into how legumes may exploit silicon accumulation to negatively impact herbivores. We demonstrated that silicification of a legumesynergises with other anti-herbivore defences to reduce performance and feeding activity of a legume-feeding insect herbivore. This was consistent across four cultivars of our study plant and, we suggest, this may be common for other legumes. Further understanding into Si dynamics in legumes, and their responses to JA, will contribute to their improved protection against herbivore threats.
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Ryalls, J.M.W., Gherlenda, A.N., Rowe, R.C., Moore, B.D. & Johnson, S.N. (2023) Silicon supplementation and jasmonate activation synergistically increase phenolic defences against a legume herbivore. Journal of Ecology, 111: 2208-2217. https://doi.org/10.1111/1365-2745.14170